Insecticide resistance and its potential mechanisms in field-collected German cockroaches (Blattodea: Ectobiidae) from Thailand

We investigated insecticide resistance profiles of field populations of the German cockroach, Blattella germanica (L.), collected from central regions of Thailand. Seven strains (PW, RB, MTH, MTS, TL, AY, and SP) were evaluated with diagnostic doses (DD; 3 × LD95 generated from a susceptible strain) of deltamethrin, fipronil, and imidacloprid using topical assays and compared with a susceptible strain (DMSC). Results showed fipronil (2–27% mortality), deltamethrin (16–58% mortality), and imidacloprid (15–75% mortality) resistance in the field strains. Synergism studies with piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) in combination with the DD of insecticides significantly increased (P < 0.05) mortality of the test insects of the field strains suggesting the involvement of P450 monooxygenase and esterase pathways of detoxification. Gel bait evaluations demonstrated that all field-collected strains were resistant to Maxforce Forte (0.05% fipronil), Maxforce Fusion (2.15% imidacloprid), and Advion Cockroach Gel Bait (0.6% indoxacarb) with mean survival times ranging from 1.87–8.27, 1.77–11.72, and 1.19–3.56 days, respectively. Molecular detection revealed that the Rdl mutation was completely homozygous in all field-collected strains except in the PW strain. Field-collected strains were screened for 3 voltage-gated sodium channel (VGSC) mutations associated with pyrethroid resistance. The L993F mutation was present in 5 strains, but no C764R and E434K mutations were detected.